Safer Code – Secure Coding In C \ C++ And More..

Subscribe To Our Feed | Follow Us On Twitter | Get Updates on Email

Alright!! In my last post about untrusted inputs, we talked about validating the data of the “integer” input parameters, checking the out parameters et cetera.This time, we’ll talk about other types of inputs. If you have written a program to take in multiple lines of strings as an input from the user, you need to make sure that the input is not tainted. It is clean and as per your expectations. For example: If your program requires an answer for a question which can be subjective, then you need to provide a string buffer good enough to get a complete answer but not large enough to crash your system or make it run out of memory. Or you need to protect your system from getting any malicious scripts being inserted.Strings are a very risky area for inputs as there is pre-defined rule for this type of validation. So, following are the points to ponder to make your code safe and secure.

1. Firstly, do use regular expressions to validate the string input. For example, ^[A-Za-z0-9]+$ specifies that the string must be at least one character long and that it can only include upper-case letters, lower-case letters, and the digits 0 through 9 (in any order). You can use regular expressions to limit which characters are allowed and to be more specific (for example, you can often limit even further what the first character can be).If you use regular expressions, be sure to indicate that you want to match the beginning (usually symbolized by ^) and end (usually symbolized by $) of the data in your match. If you forget to include ^ or $, an attacker could include legal text inside their attack to bypass your check.
2. Now, if your program needs more variety of input and the above point doesn’t fulfil the requirements then you need to make a bit more complicated regular expressions. If the data is a filename (or will be used to create one), be very restrictive. Ideally, don’t let users choose filenames, and if that won’t work, limit the characters to small patterns such as ^[A-Za-z0-9][A-Za-z0-9._\-]*$. You should consider omitting from the legal patterns characters like “/”, control characters (especially newline), and a leading “.”Similarly, you need to take care for email strings, locale specific strings. UTF-8 encoding characters et cetera. In most of the programs, complex regular expressions are good enough to validate a string. But in certain cases, a malicious input containing some script code can spoil the fun.
3. If your program faces HTML tags or script related instructions in the input, the input should be rejected immidiately or your program might get infected with self executing malicious code. This technique is generally used in Cross Site scripting attack. (XSS attack). These problems are especially a problem for Web applications. Now, you need to again take care not to validate any input which looks like an HTML tag. The easiest way is to use above mentioned regular expressions which won’t allow the entry of ‘<’ or ‘>’ character. But if you must support some of the HTML tags like <a href=> etc, please validate them exclusively by filtering the whole string using a regex like ^(http|ftp|https)://[-A-Za-z0-9._/]+$. A pattern that allows some more complex patterns is: ^(http|ftp|https)://[-A-Za-z0-9._]+(\/([A-Za-z0-9\-\_\.\!\~\*\'\(\)\%\?]+))*/?$
4. For more complex strings, like reading a data file, regular expressions, again, prove useful but the ideal way is to break the file into multiple chunks rather than reading it in one complete string.

To Keep your String input kept in well defined range or buffer, make sure that your program terminates it with a NULL character. This will ensure that even if a large buffer is inserted using the input, the sting will get truncated as soon as the buffer gets full and it will be protected from buffer overflow.

© Safer Code | Validating Untrusted String Inputs

Liked this post? Get FREE Updates Subscribe to RSS feed Or Enter Your E-mail ID below

Subscribe To Our Feed | Follow Us On Twitter | Get Updates on Email

Thousands of sites around the interwebs are devoted to programming and producing code. But there is something missing. This “something” is actually the most important piece of the puzzle. This piece is about how “safe” and “efficient” your code is. There are programmers all over, but a very small minority is worried about finding all the loopholes in their code. Infact, most don’t even know there could be loopholes even as they start writing their program (more on this very soon ). And many times, you’d see people creating a jet fighter for something that could be solved with a bicycle (although the pace would be vice-versa).

The problem here is that there are many things that are not taught in the schools, the knowledge might be out there on the internet, but either it is fuzzy, not properly explained or is plagued by just too much of information overload. You cannot imagine yourself (and everyone else around you) to sit in a week-long class and emerge a champion programmer. It has to seep in gradually.

So, this is an attempt to solve the problem. We’ll bring you the concepts to make your code a fortress. We’ll bring them at a gradual pace that gives you time to learn, understand, ask questions and imbibe them into your daily routines. The problems and solutions would range from the very basics and trivia to the most advanced. We’d concentrate mostly on examples through C/C++ with a bit of JAVA and others interspersed here and there when needed but most concepts learned could be as well applied to any language. We’ll not tell you how to program, we assume you already know, but we’ll tell you how to program efficiently and securely.

So, if you are a college goer, or a fresher just into the corporate world, or an experienced professional, we have something for you all, to make you so capable that you can take a running program and re-write it so that it runs for years without crashing, being exploited to death, or taking a ton of memory or cycles.

Enough talking now. As Linus once said “Talk is cheap. Show me the code.”. So, lets begin…

© Safer Code | And So It Begins…

Liked this post? Get FREE Updates Subscribe to RSS feed Or Enter Your E-mail ID below